Up to now, for 3GPP LTE-A systems, channel feedback mechanisms for support of Downlink Coordinated Multi-Point (DL CoMP) are still under discussion and formulation. The existing solutions for channel feedback to enable the Downlink Coordinated Multi-Point transmission include: 1) Precoding Matrix Index (PMI) feedback; 2) Uplink (UL) sounding feedback; and 3) physical channel matrix feedback. The above feedback methods, however, have the following defects:
1) PMI feedback is more efficient in single Base Station (BS) transmission modes. However, in multi-cell transmission, the PMI feedback may largely limit the freedom of precoding operation of involved multiple cells.
2) UL sounding feedback is more efficient in Time Division Duplex (TDD) mode, but it is difficult to be applied in Frequency Division Duplex (FDD) mode. Additionally, asymmetric interference distribution among Downlink (DL) and Uplink (UL) will damage the accuracy of channel information estimation;
3) Physical channel matrix feedback will lead to too large feedback overhead. For example, the feedback solutions as described by NTT DoCoMo in Proposal R1-094244 in 3GPP RAN WG1 #58 bits meeting, no matter the channel vector quantization based method (case 1) or the channel element quantization based method (case 2), will lead to a large feedback overhead (e.g., up to 26.6 bits/subband/cell in case 1 and up to 53.3 bits/subband/cell in case 2), which may be unacceptable in practical systems.